1. Field of the Invention
The present invention relates to a method for manufacturing a luminescent material, a luminescent material manufactured by the manufacturing method, and a display substrate and a display apparatus having the luminescent material. More particularly, it relates to a method for manufacturing a luminescent material in which the surfaces of phosphor particles are coated with glass, a luminescent material manufactured by the manufacturing method, and a display substrate and a display apparatus having the luminescent material.
2. Description of the Related Art
In recent years, a field-emission display (hereinafter, referred to as a FED) has received attention as a display which has achieved a low profile. In a conventional TV cathode-ray tube display, an electron beam can sufficiently penetrate into a luminescent material particle because its high acceleration voltage (from 30 Kv), resulting in a wide-range excitation. Namely, the luminescent material can receive the excitation energy (energy of the electron beam) over the wide-range area thereof, resulting in less burden on the luminescent material. However, in the FED, since the luminescent material is excited at a low acceleration voltage (from 10 Kv), the penetration distance of an electron beam is shortened. Accordingly, the luminescent material must receive the excitation energy within the narrow-range area thereof, resulting in more burden on the luminescent material.
FIG. 1 is a view for schematically illustrating the penetration behavior of an electron beam into a luminescent material, i.e., a phosphor. The electron beam (thermoelectrons) emitted into the phosphor consecutively undergoes collision and scattering in the phosphor to lengthen the mean free path into the deep recesses of the phosphor because of its high energy when emitted at a high acceleration voltage (A of FIG. 1). Accordingly, the area of the phosphor to be excited is expanded, resulting in an increase in emission luminance. In contrast, when emitted at a low acceleration voltage (B of FIG. 1), the scattering area of the thermoelectrons in the phosphor cannot be expanded. Accordingly, the area of the phosphor to be excited is narrowed, resulting in a reduction in emission luminance.
In the FED, since the acceleration voltage is low, a large amount of current must be passed therethrough in order to obtain sufficient luminance. However, most luminescent materials have low electric conductivity. Therefore, passage of a large amount of current therethrough promotes electrification on the surfaces of the luminescent material particles. This electrification then causes an increase in temperature to accelerate the degradation of the luminescent material. Thus, the conditions of low acceleration voltage and high current under which the luminescent material in the FED is excited act as adverse conditions accelerating the degradation of the luminescent material. For the FED, the overcoming of the degradation is an important problem.
Further, in the FED, the luminescent surface is formed by a method in which a luminescent material is applied onto a glass substrate, or the like. For this reason, the luminescent material is required to be efficiently obtainable in keeping with a trend toward a larger-size a screen. Therefore, such efficient acquisition of the luminescent material becomes another important problem.
Incidentally, the luminescent materials to be used for a display include an oxide-based luminescent material and a sulfide-based luminescent material. Both have their respective advantages and disadvantages. For the oxide-based luminescent material, the life characteristics are superior, but the luminous efficiency and the emitted color are inferior. Whereas, for the sulfide-based luminescent material, the life characteristics are inferior, but the luminous efficiency and the emitted color are superior.
If the oxide-based luminescent material is used in the FED which regards the overcoming of the degradation of the luminescent material as a critical problem as described above, no problem occurs with respect to the degradation. But, on the other hand, there occur problems in that the power consumption is increased because of the low luminous efficiency when the luminescent material is used as a display, and in that the emitted color is inferior, and the color reproducibility is not excellent.
In contrast, the sulfide-based luminescent material is an attractive material as a luminescent material for the FED because of its excellent luminous efficiency and emitted color. Therefore, if the life characteristics of the sulfide-based luminescent material can be improved, it is possible to improve the performance thereof as a display for the FED correlatively with its luminous efficiency and emitted color. Thus, in the FED, improvement of the life of the luminescent material, or the efficient acquisition of the luminescent material having an improved life has been long awaited for the sulfide-based luminescent material.
The deterioration of the luminescent material by an electron beam can be inhibited by the following methods:
i) A method in which a mixed type luminescent material including a conductive substance mixed therein, or a deterioration-resistant luminescent material mixed therein is used; and
ii) A method in which the luminescent material particles are surface-coated with a chemically stable substance (typical examples of such a substance may include glass).
As for the method (ii), the following methods have been adopted: a method in which water glass is diluted with an aqueous alkaline solution for coating when the luminescent material of an alkaline-earth metallic sulfide having a hydrolytic property such as calcium sulfide (CaS) is surface-coated with water glass (see Japanese Laid-Open Patent Publication No.Sho 58-80375, below, referred to as a first method); a method in which in coating the phosphor surface with water glass, an aqueous mixed solution of water glass, aluminium sulfate, and sodium hydroxide is used for coating (see Japanese Laid-Open Patent Publication No.Hei 7-312287, blow, referred to as a second method); and further, a method in which a suspension obtained by adding a luminescent material and water glass in an aqueous barium acetate solution is supplied into an inverted cathode-ray tube (CRT) from the neck portion to deposit the luminescent material on the faceplate (below, referred to as a third method).
However, with the method (i), there have occurred the following problems: it is difficult to uniformly disperse the mixed substance in a slurry or a paste according to a difference in specific gravity, particle size, and particle shape in forming a fluorescent surface by a slurry method or a printing method; all the luminescent material particles are not in contact with the mixed substance, so that sufficient mixing effects cannot be exerted; and the like.
Further, out of the methods (ii), with the first method, i.e., the method in which water glass is diluted with an aqueous alkaline solution for coating, there have occurred the following problems: when this method is applied to a luminescent material containing zinc sulfide as a matrix, a glass film is formed in an islands structure, or the film thickness distribution is not uniform, thereby to cause the chemical reaction with a residual gas on the luminescent material surface, and particularly the oxidation of the surface and the separation of sulfur for the sulfide-based luminescent material, resulting in accelerated degradation, and no luminance degradation inhibiting effect; and the like. With the second method, i.e., the method in which an aqueous mixed solution of water glass, aluminium sulfate, and sodium hydroxide is used for coating, there have occurred the following problems: the aluminium included in the glass tends to form a solid solution with the zinc sulfide, so that a change in emitted color and a reduction in luminous efficiency may be caused; and the like.
Still further, with the third method, i.e., the method in which a luminescent material is deposited on the CRT faceplate from a suspension obtained by adding a luminescent material and water glass in an aqueous barium acetate solution, there has occurred the following problem: although a glass film can be uniformly formed on each luminescent material particle, much time is taken for the deposition and the treatment of an undesired solution, so that the luminescent material cannot be obtained efficiently.
The present invention has been completed in view of such circumstances. It is therefore an object of the present invention is to provide a manufacturing method whereby when the surfaces of phosphor particles are coated with a chemically stable substance (glass), it becomes possible to uniformly coat the surface of every phosphor particle with the substance (glass), and it is possible to efficiently obtain a luminescent material made up of such particles. In addition, it is further objects to obtain a luminescent material by such a manufacturing method, and to provide a display substrate and a display apparatus having the same.
Specifically, in accordance with a first aspect of the present invention, there is provided a method for manufacturing a luminescent material which comprises, the steps of: adding a phosphor in the form of a solid solution comprising a sulfide or an oxysulfide, and a luminescent center substance incorporated in the sulfide or the oxysulfide in an aqueous solution containing water glass with stirring; adding the resulting aqueous solution to an aqueous solution of a metallic salt with stirring; forcedly extracting particles of the phosphor from the resulting aqueous solution; and drying the particles of the phosphor extracted.
In accordance with further aspects of the present invention, there is provided a manufacturing method of each of a luminescent material according to the first aspect of the present invention, wherein the sulfide is zinc sulfide, and the luminescent center substances are silver and chlorine; a luminescent material according to the first aspect of the present invention, wherein the sulfide is zinc sulfide, and the luminescent center substances are silver and aluminium; a luminescent material according to the first aspect of the present invention, wherein the sulfide is zinc sulfide, and the luminescent center substances are silver, aluminium, and copper; a luminescent material according to the first aspect of the present invention, wherein the sulfide is zinc sulfide, and the luminescent center substances are copper and aluminium; a luminescent material according to the first aspect of the present invention, wherein the sulfide is zinc sulfide, and the luminescent center substances are copper, gold, and aluminium; and a luminescent material according to the first aspect of the present invention, wherein the oxysulfide is yttrium oxysulfide, and the luminescent center substance is europium.
In accordance with still further aspects of the present invention, there are provided a luminescent material manufactured by the same method, and a display substrate using the luminescent material, and a display apparatus using the same.
In accordance with the present invention, a luminescent material is manufactured through the steps of adding a phosphor in the form of a solid solution comprising a sulfide or an oxysulfide, and a luminescent center substance incorporated in the sulfide or the oxysulfide in an aqueous solution containing water glass with stirring; adding the resulting aqueous solution to an aqueous solution of a metallic salt with stirring; forcedly extracting particles of the phosphor from the resulting aqueous solution; and drying the particles of the phosphor extracted. Therefore, glass is uniformly coated around every phosphor particle. In addition, it is possible to efficiently manufacture a luminescent material made up of such particles.
Further, in the luminescent material manufactured through such manufacturing steps, the glass coated around the phosphor particles is uniform. Accordingly, the chemical reaction with a residual gas on the surface of the luminescent material, particularly for a sulfide-based luminescent material, oxidation of the surface and separation of sulfur will not occur, resulting a longer life as the luminescent material.
Still further, in the display substrate having the luminescent material manufactured through the manufacturing steps of the present invention, the luminescent surface can be formed in a short time. Therefore, by using such a substrate, it is possible to improve the productivity of the display apparatus.
Furthermore, the display apparatus having the luminescent material manufactured through the manufacturing steps of the present invention is usable for a long period as a display apparatus because of the long life of the luminescent material.